This invention relates to a process of manufacturing a porous resin molded product containing continuous holes, which is used for filtering media, gas dispersive materials, and for mold materials, and further relates to a casting mold, which contains continuous holes, suitable for slip discharge casting.
Porous resin molds containing continuous holes have generally been used as filtering media, gas dispersive materials, and for mold materials. When such molded products are used as filtering media, accurate reproduction of pore diameter and porosity are essential. At the same time, the porous resin molded products containing continuous holes should fully meet the requirements of highly important characteristics such as mechanical properties, dimensional stability, and durability.
The methods for manufacturing porous resin molded products containing continuous holes adopted so far, however, have failed to meet the requirements because of problems as described in the following text. Consequently, the obtained porous resin molded products containing continuous holes could be used in a limited range only. When made by the method which uses a non-organic powder, for example, the obtained molded product is very brittle and is often chipped or cracked at remolding. As it requires high-temperature baking, large sized molded products become cracked in almost all cases. In the metal powder baking method, even dispersion of the metal powder is difficult to achieve and pore diameter and porosity tend to be locally non-homogeneous due to transfer and gathering of the metal powder upon baking. This makes it hard to reproduce the pore diameter and porosity with high accuracy. The method which uses plaster or cement is very handy since molding can be performed by casting. Accordingly, plaster is used popularly as a material for making porous resin molds containing continuous pores. Molds made by using materials that are reactive to hydration, such as plaster and cement, however, have such problems as low durability and low resistance to chemicals. To solve these problems, an improved method comprising addition of a synthetic resin or an emulsion or solution of a synthetic resin has been studied. Since even dispersion of synthetic resin or the emusion solution is difficult, the improved method is still subjected to major problems such as inferior reproduction of pore diameter and porosity, and contraction of the mold.
Besides the manufacturing methods as mentioned above, another process which uses epoxy resin of water dispersion system is shown in Publication No. 53-2464 of Japanese Patent. As an aliphatic polyamide is used as the hardener, a compound of high fluidity for molding is not obtained unless dispersed with a large quantity of water. Accordingly, it takes rather a long curing time from casting to remolding and the mold is greatly contracted upon heating and evaporation, which causes a serious problem in dimensional stability. When a filter is added for adjustment of the molding mixture to solve the problem, fluidity of the mixture is lowered making casting difficult, the mechanical characteristics of the obtained mold are lowered, and the weight is also increased. Accordingly, the method is applicable to manufacturing molds of limited shapes and dimensions only.
An improved version of the method of Japanese Patent Publication No. 53-2464 is also shown in Provisional Publication No. 59-71339. The improved method is characterized by the use of aliphatic polyamide as the condensation product of a mono-fatty acid and amine used in addition to the polymerized aliphatic polyamide used as the hardener in Publication No. 53-2464, so as to obtain a porous material containing continuous holes of a desired average pore diameter. As the condensation product of fatty acid and amine is used as a hardener, as mentioned above, the epoxy resin slip with these kinds of hardeners must be water in oil type of emulsion to gain a certain viscosity capable of casting and accordingly, the added quantity of water reaches the high percentage of the epoxy resin slip. This extends the curing time and the mold is greatly contracted, which causes a serious problem in dimensional stability. Another problem is still left unsolved in view of production of a porous article containing continuous holes at a high reproduction accuracy.
In Japanese Patent Publication No. 51-46131, a manufacturing method is disclosed, which is characterized by the use of epoxy resin and a specific and sole emulsifier. As is seen hereinafter in the examples for comparison, this method has a problem of uneven pore diameter due to unstable dimensions of the obtained porous material. When a filler is used in addition, the emulsion is subjected to water release in the middle of curing because a sole emulsifier does not function to stabilize the emulsion.
Japanese Patent Publication No. 55-19723 shows a method to use polymethyl methacrylate of powder form in the main. This method is also subjected to such problems as contraction due to unstable dimensions of the obtained mold as polymethyl methacrylate is used for emulsion polymerization, and lower mechanical properties and heat resistance than the method with epoxy resin. The description of this manufacturing process includes no statement of the embodiment where any filter is used.
On the other hand, slip discharge casting is a method adapted to make hollow molds. By this method, slip is supplied into the cast space of a casting mold which can be separated and has continuous pores, the slip is pressed to discharge water content in the slip outside the casting mold, and the solid matter is deposited inside the casting mold. Then the casting mold is separated to obtain a hollow mold. If the casting mold for this method is made of plaster, the durability and resistance to chemicals are inferior. If the mold is made by the method of Japanese Patent Publication No. 53-2464, on the other hand, the problem of unstable dimensions results. For manufacturing a mold, therefore, either method is unfavorable and a more suitable casting mold to slip discharge casting has been desired.